/Engine/Source/Runtime/Engine/Classes/Kismet/KismetMathLibrary.h
Transform a direction vector - will not take into account translation part of the FMatrix. If you want to transform a surface normal (or plane) and correctly account for non-uniform scaling you should use TransformByUsingAdjointT. (Assumes Matrix represents a transform)
static FVector4 Matrix_TransformVector ( const FMatrix & M, FVector V )